Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
1999-10-01
2002-07-30
Huber, Paul W. (Department: 2653)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S047230
Reexamination Certificate
active
06426934
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to a reproducing apparatus and a signal processing apparatus each having a filter for eliminating cross modulation distortion if a cross modulation noise is caused by poor compatibility between an amplifier and a speaker connected to a reproducing apparatus for reproducing a digital signal of which quantization bit having high-frequency band is one bit.
The PCM (Pulse Code Modulation) signal for use in the compact disc (CD) is a digital signal which is limited in band to 22 KHz. As shown in
FIG. 1
, an RF signal reproduced by an optical pickup
51
from an CD
50
is converted by a signal processor
52
into a PCM signal. The PCM signal is then filtered by a digital filter
53
to steeply eliminate sampling noise higher than 22 KHz. The resultant signal is then converted by a D/A converter
54
into an analog signal to be supplied to an analog low-pass filter
55
.
Recently, a digital audio signal has become a focus of attention which consists of one bit data obtained by performing delta sigma (&Dgr;&Sgr;) modulation on an analog audio signal. This digital audio signal is hereafter referred to as a direct stream digital (DSD) signal. Compared with a conventional PCM audio signal having format of sampling frequency 44.1 KHz and data word length 16 bits for example, the DSD signal comprises a very high sampling frequency such as 64 times 44.1 KHz and a short data word length such as one bit for example, featuring a wide transmittable frequency band. In addition, in the audio band, which is low enough by performing sampling on such a high over-sampling frequency as 64 times 44.1 KHz, the DSD signal can ensure a high dynamic range. These features allow this signal to be applied to recording and data transmission for example of high tonal quality.
The DSD signal is also a one-bit pulse train having a signal component ranging to 1.4 MHz. Simply passing this signal through a low-pass filter provides an analog audio signal. As shown in
FIG. 2
for example, an RF signal is reproduced through an optical pickup
61
from an optical disc
60
on which a DSD signal is recorded and the RF signal is converted by a signal processor
62
into the DSD signal. This DSD signal is then passed through a low-pass filter
63
to obtain an analog audio signal at an output terminal
64
.
Referring to
FIG. 3
, there is shown a constitution of a &Dgr;&Sgr; modulator for obtaining the above-mentioned one-bit quantization data. This &Dgr;&Sgr; modulator comprises an adder
71
, an integrator
72
, a quantizer
73
, and a delay circuit
75
. The output of the adder
71
is supplied to the integrator
72
. The output of the integrator
72
is supplied to the quantizer
73
. The output of the quantizer
73
is taken out at an output terminal
74
and, at the same time, converted by the delay circuit
75
into a negative code to be fed back to the adder
71
, in which the negative coded is added to the analog audio signal supplied from an input terminal
70
. The output of the adder
71
is then integrated by the integrator
72
. The output of the integrator
72
is quantized by the quantizer
73
in every sampling period. Consequently, one-bit quantization data can be obtained at the output terminal
74
.
Meanwhile, in the case of the above-mentioned PCM signal, there is no fear that the components of higher than 22 KHz leak into devices in the succeeding stage because these components are steeply eliminated by the digital filter
53
. In the case of a DSD signal, however, if the blocking by the analog low-pass filter
63
is insufficient, unnecessary high-frequency components may leak in the succeeding stage such as an amplifier or a speaker to cause cross modulation distortion, thereby deteriorating tone quality. If the above-mentioned undesired high-frequency components leak in a high amplitude into a speaker capable of reproducing with fidelity high-frequency tones of up to 100 KHz for example, the speaker may be damaged.
The undesired high-frequency components include a noise component that is shifted to the high-frequency side by a noise shaper when a DSD signal is recorded onto a recording medium such as an optical disc, a hard disc, or a magnetic tape. Currently, the degree of the noise shapers for generating a DSD signal is mainly 3 to 7. The low-pass filter must eliminate the high-frequency components, namely these noise components. However, in the case of a DSD signal, the characteristics of the noise shaper can be set at will in the recording system, so that it is unclear how much noise exists in which band. For example, there is an apparent difference in the quantity of undesired high-frequency component between the noise characteristic obtained by a 3rd-order noise shaper shown in FIG.
4
and the noise characteristic obtained by a 7th-order noise shaper shown in FIG.
5
. The low-pass filter on the reproducing apparatus side cannot recognize this difference. Namely, when recording a DSD signal on an optical disc, a hard disc, or a magnetic disc for example, the reproducing apparatus side cannot determine whether the recording is through a 3rd-order noise shaper or a 7th-order noise shaper. Further, when a higher-order noise shaper is developed in the future, the currently available low-pass filters are not enough to block undesired high-frequency components.
Also, because DSD signal range as wide as 1.4 MHz, simply lowering the cutoff signal fc of the low-pass filter cannot fully make the most of the DSD signal performance. Consequently, it is hard to reproduce a DSD signal in the same manner as PCM signal reproduction by means of one low-pass filter.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a digital signal reproducing apparatus that allows the selection of an appropriate low-pass filter according to the configuration of a succeedingly connected device and the high-frequency component of a digital signal consisting of one-bit quantization data to be reproduced and, therefore can prevent the undesired high-frequency component from leaking due to insufficient blocking into the succeeding stage, thereby preventing the cross modulation distortion from occurring in the succeeding stage.
It is another object of the present invention to provide a digital signal processing apparatus that allows selection of an appropriate low-pass filter according to the high-frequency component of a one-bit digital signal and can supply a signal with the occurrence of cross modulation distortion prevented to an amplifier.
In carrying out the invention and according to one aspect thereof, there is provided a reproducing apparatus for reproducing a recording medium recorded with a digital audio signal composed of a quantization one-bit data sampled at a sampling frequency fs (Hz) sufficiently higher than an audio band, comprising: a reproducing means for reproducing the digital audio signal from the recording medium; a signal processing means for performing predetermined signal processing on the digital audio signal reproduced by the reproducing means; a low-pass filter means having a plurality of different frequency characteristics with cutoff frequency above the audio band; and a selecting means for selecting one of the plurality of different frequency characteristics of the low-pass filter means according to a user's operation.
In carrying out the invention and according to another aspect thereof, there is provided a reproducing apparatus for reproducing a recording medium recorded with a digital audio signal composed of quantization one-bit data sampled at a sampling frequency sufficiently higher than an audio band, comprising: a reproducing means for reproducing the digital audio signal from the recording medium; a signal processing means for performing predetermined signal processing on the digital audio signal reproduced by the reproducing means; a low-pass filter means having a plurality of different frequency characteristics with cutoff frequency above the audio band; a high-frequency comp
Huber Paul W.
Maioli Jay H.
Sony Corporation
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